Emissions from internal combustion engines, including diesel engines, are limited by legislation put in place by governments worldwide. Manufacturers are seeking to meet these legislated requirements through a combination of engine design and exhaust gas after-treatment. The exhaust systems used to carry out exhaust gas after-treatment commonly comprise a series of catalysts and/or filters that are designed to carry out certain reactions that reduce the proportion of exhaust gas species limited by such legislation.
A diesel engine exhaust stream is a heterogeneous mixture which contains not only gaseous emissions such as carbon monoxide (“CO”), unburned hydrocarbons (“HC”) and nitrogen oxides (“NOx”), but also condensed phase materials (liquids and solids) which constitute the so-called particulates or particulate matter. Often, catalyst compositions and substrates on which the compositions are disposed are provided in diesel engine exhaust systems to convert certain or all of these exhaust components to innocuous components. For example, diesel exhaust systems can contain one or more of a diesel oxidation catalyst, a soot filter and a catalyst for the reduction of NOx.
The total particulate matter emissions of diesel exhaust streams include a solid, dry, carbonaceous fraction, a so-called soot fraction. This dry carbonaceous matter contributes to the visible soot emissions commonly associated with diesel exhausts.
One key after-treatment technology in use for high particulate matter reduction is the diesel particulate filter. There are many known filter structures that are effective in removing particulate matter from diesel exhaust streams, such as honeycomb wall-flow filters, wound or packed fiber filters, open cell foams, sintered metal filters, etc. However, ceramic wall flow filters, described below, receive the most attention. These filters are capable of removing over 90% of the particulate material from diesel exhaust streams. The filter is a physical structure for removing particles from exhaust streams, and the accumulating particles will increase the back pressure from the filter on the engine. Thus the accumulating particles have to be continuously or periodically burned out of the filter to maintain an acceptable back pressure. Unfortunately, the carbon soot particles require temperatures in excess of 500° C. to burn under oxygen rich (lean) exhaust conditions. This temperature is higher than what is typically present in diesel exhaust streams.
Accordingly, it is necessary to actively burn the accumulated soot in order to promote filter regeneration. One form of active filter regeneration is to intermittently introduce additional hydrocarbon fuel into the exhaust gas and to combust this in order to increase the filter temperature. Combustion of the additional hydrocarbon fuel can be effected on the filter itself by coating the filter with a suitable combustion-promoting catalyst. A suitably catalysed filter is often referred to as a catalysed soot filter or CSF.
During active regeneration the CSF may need to reach temperatures of approximately 600° C. to permit particulate matter to be removed (combusted) at a sufficient rate. However, if during an active regeneration event, a period of low exhaust gas flow occurs, e.g. when the engine/vehicle is caused to idle, the reduced gas flow prevents heat from being removed from the CSF. This can result in parts of the filter reaching temperatures in excess of 1000° C. Such high temperatures can cause two major problems. Firstly, the catalyst can sinter, reducing its surface area and as a consequence catalyst activity is lost. Secondly, high thermal gradients can occur in the substrate leading to mechanical stress caused by differences in thermal expansion. Under extreme conditions the thermal gradients and stresses can cause substrates to crack thereby resulting in a failure of the integrity of the CSF.
Therefore, the challenge is in controlling the active regeneration of the CSF so that it can reach temperatures sufficiently high to remove particulate matter but not so high as to cause damage to the catalyst and/or the filter substrate.
As noted above diesel exhaust streams also contain NOx. A proven NOx abatement technology applied to stationary sources with lean exhaust conditions is Selective Catalytic Reduction (SCR). In this process, NOx is reduced with ammonia (NH3) to nitrogen (N2) over a catalyst typically composed of base metals. The technology is capable of NOx reduction greater than 90%, and thus it represents one of the best approaches for achieving aggressive NOx reduction goals. SCR provides efficient conversions of NOx as long as the exhaust temperature is within the active temperature range of the catalyst.
Separate substrates, each containing catalysts to address discrete components of the exhaust can be provided in an exhaust system. However, use of fewer substrates is desirable to reduce the overall size of the system, to ease the assembly of the system, and to reduce the overall cost of the system. One approach to achieve this goal is to coat the soot filter with a catalyst composition effective for the conversion of NOx to innocuous components. With this approach, the catalysed soot filter assumes two catalyst functions: removal of the particulate component of the exhaust stream and conversion of the NOx component of the exhaust stream to nitrogen.
Coated soot filters that can achieve NOx reduction goals require a sufficient loading of SCR catalyst composition on the soot filter. The gradual loss of the catalytic effectiveness of the compositions that occurs over time through exposure to certain deleterious components of the exhaust stream augments the need for higher catalyst loadings of the SCR catalyst composition. However, preparation of coated soot filters with higher catalyst loadings can lead to unacceptably high back pressure within the exhaust system. Coating techniques that allow higher catalyst loadings on the wall flow filter, yet still allow the filter to maintain flow characteristics that achieve acceptable back pressures are therefore desirable.
An additional aspect for consideration in coating the wall-flow filter is the selection of the appropriate SCR catalyst composition. First, the catalyst composition must be durable so that it maintains its SCR catalytic activity even after prolonged exposure to higher temperatures that are characteristic of filter regeneration. For example, combustion of the soot fraction of the particulate matter often leads to temperatures above 700° C. Such temperatures render many commonly used SCR catalyst compositions such as mixed oxides of vanadium and titanium less catalytically effective. Second, the SCR catalyst compositions preferably have a wide enough operating temperature range so that they can accommodate the variable temperature ranges over which the vehicle operates. Temperatures below 300° C. are typically encountered, for example, at conditions of low load, or at startup. The SCR catalyst compositions are preferably capable of catalyzing the reduction of the NOx component of the exhaust to achieve NOx reduction goals, even at lower exhaust temperatures.
U.S. Pat. No. 8,617,476 discloses a honeycomb filter characterised by the amount of zeolite supported on the channel walls and the thermal conductivity of the walls.
U.S. Pat. No. 8,398,925 discloses a particulate filer substrate for an internal combustion engine. The filter substrate is coated with a washcoat having regions of different densities.
WO2005016497 discloses an exhaust treatment system.
US2012/0247092 discloses a multi-component filter for emission control.
US2014/0140899 discloses a catalysed particulate filter.
WO2011140248 discloses catalysed soot filters and emission treatment systems.
U.S. Pat. No. 5,221,484 discloses a catalytic filtration system and method.
Accordingly, it is desirable to provide an improved wall-flow monolith and/or tackle at least some of the problems associated with the prior art or, at least, to provide a commercially useful alternative thereto.